The demand for data throughput growth leads to increasingly widespread use of various radio communication systems operating in the millimeter wave range. Such increase is associated, on the one hand, with a wide frequency bandwidth available for use in said range, and on the other hand, with significant technological advances made over the past few decades, allowing to create modern, effective and cost-efficient (in terms of large-scale production) transceivers operating in frequency ranges from 30 GHz to over 100 GHz. Modern millimeter wave radio communication systems include, without limitation, radio-relay stations providing point-to-point and point-to-multipoint communications, car radars, wireless local area communication networks, etc.
The effectiveness of millimeter wave communication systems is determined largely by characteristics of antennas used in said systems. Such antennas generally should have a high gain value, and consequently, should form a narrow radiation pattern beam. In this case, the antennas provide effective (i.e. with maximum throughput) signal transmission over long distances, but said antennas also require precise alignment of narrow beams between two radio communication stations.
The requirement for high gain value is determined by a small wavelength of radiation in said frequency range, which leads to difficulties in transmitting a signal over long distances using antennas with insufficient gain values. Furthermore, in said frequency range, the effect of weather conditions and atmospheric absorption is high (e.g., in the frequency range of about 60 GHz, the effect of oxygen spectral line absorption is high, leading to additional signal attenuation at 11 dB/km).
Known configurations of millimeter wave antennas providing high gain include antenna arrays (including slot antenna arrays implemented in a metal waveguide), reflector antennas (e.g., parabolic and Cassegrain antennas), various types of lens antennas (e.g. thin lenses with separated feed, Fresnel lenses, Luneburg lenses, artificial lenses from a reflect arrays). In order to provide a high gain value, the dimensions of radiating aperture in all such antennas greatly exceed the operating wavelength. A review of various aperture antenna configurations can be found, e.g., in Y. T. Lo, S. W Lee, Antenna Handbook. Volume II: Antenna Theory, Springer, 1993, pp. 907.
Advances in aperture antenna technology are directed at several areas. On the one hand, high gain value is provided easily by enlarging the radiating aperture, which primarily requires improving the precise manufacturing technology of reflector antennas mirrors, lenses and other secondary focusing devices of large sizes. On the other hand, when using a fixed aperture size, the increase in gain value is provided by increasing the aperture efficiency of the antenna, by improving impedance matching, and by increasing the radiation efficiency. For that purpose, a diversity of new and improved aperture antenna arrangements has been developed.
The increase in gain value of an aperture antenna is generally provided by forming a more effective amplitude-phase distribution at the equivalent aperture of the antenna. For example, in horn-lens antennas, it can be accomplished by inserting a dielectric lens into the horn that allows providing flat wave front of the radiation. One of the embodiments of a horn-lens antenna is disclosed, in particular, in U.S. Pat. No. 6,859,187. However, despite the fact that said antennas provide an increase in gain value, they are quite large (i.e. axially large), difficult to manufacture, and consequently, expensive to produce.
Therefore, in the new aperture millimeter wave antenna structures, it is important to provide ease of implementation and installation, as well as a wide radiation frequency band. One of the most promising antenna types that provides high gain value in wide frequency range and has a simple construction is a lens antenna with an integrated antenna element (see, e.g., W. B. Dou and Z. L. Sun, “Ray Tracing on Extended Hemispherical and Elliptical Silicon Dielectric Lenses,” International Journal of Infrared and Millimeter Waves, Vol. 16, pp. 1993-2002, No. 1L, 1995, and A. Karttunen, J. Ala-Laurinaho, R. Sauleau, and A. V. Raisanen, “Reduction of Internal Reflections in Integrated Lens Antennas for Beam-Steering,” Progress In Electromagnetics Research, Vol. 134, pp. 63-78, 2013).
A lens antenna with an integrated antenna element is known from U.S. Pat. No. 5,706,017, titled “Hybrid Antenna Including a Dielectric Lens and Planar Feed”. The increase in gain value in such antenna is provided by using a lens of a specific shape, said lens focusing the radiation in a certain spatial direction from the primary antenna element that is installed in the focal plane on the surface of the lens. The shape of the collimating part of the lens is calculated directly from the dielectric properties thereof, in particular, from the dielectric constant (∈>1). The canonical shape of the collimating part of the lens in the disclosed antennas is a hemiellipsoid of revolution or a hemisphere. A non-collimating part of the lens is formed as an extension having various shapes and required dimensions. In this device, the object of precisely positioning the antenna element with respect to the lens focus is further achieved by placing the primary antenna element directly on the flat surface of the lens, thus providing simplicity of design and assembly of the antenna.
The lens antenna disclosed in U.S. Pat. No. 5,706,017 provides beam scanning by using an array of switchable primary antenna elements. This is made possible due to the property of the lens antenna allowing for angular deflection of the beam with respect to the axis of the lens when the primary antenna element is displaced along the flat surface of the lens, on which said antenna element is placed. Beam scanning is used for simplification and automation of beam adjustment in radio-relay point-to-point communication systems, which is a crucial objective in developing aperture antennas due to the very narrow beam of the radiation pattern.
The lens antenna 1 of U.S. Pat. No. 5,706,017 is shown in FIG. 1. Generally, the lens antenna 1 comprises a lens 2 and an antenna element 3, which is a primary antenna element. The lens 2 consists of a collimating part 4 and an extension part 5. The collimating part 4 is integrally formed with the extension part 5, and the parts 4 and 5 of the lens 2 are made of a dielectric material. The collimating part 5 of the lens 2 comprises a substantially flat surface 6 crossed by the axis of the collimating part 4 of the lens 2, and the antenna element 3 is rigidly fixed on the surface 6. The advantages of such antenna include easy and low-cost manufacturing, as well as convenient assembly and positioning of the primary antenna element 3 at a certain position with respect to the focus of the lens 2.
In order to focus the radiation from the primary antenna element 3 in a certain direction, the collimating part 3 of the lens 2 has an elliptic (or quasi-elliptic) shape with eccentricity inversely proportional to the refraction index of the lens material. The extension part 5 of the lens can have various shapes, e.g. a cylindrical shape with thickness equal to the focal length of the ellipsoid of revolution. If the required antenna diameter is small, the lenses can have modified shapes, e.g. hemispherical shape, hyperhemispherical shape, or elliptic shape with modified eccentricity.
In the lens antenna of U.S. Pat. No. 5,706,017, the primary antenna element is a planar log-spiral antenna. The advantages of such antenna include a wide frequency bandwidth and the possibility of connection a detector element between the antenna arms. However, the directivity of the spiral antenna is defined by the size thereof, which is calculated based on bandwidth requirements. This leads to difficulties in optimizing directivity of the spiral antenna for effective illumination of a dielectric lens of a specific geometry, and consequently, to difficulties in maximizing directivity of the whole lens antenna. Furthermore, such antenna is rather sensitive to imperfections during manufacturing and has quite large back-to-front radiation ratio when installed on the lens.
In some known lens antenna devices with certain types of planar integrated antenna elements, improvements are directed towards increasing gain value by special modifications of the lens shape.
Said object was addressed, e.g., in the antenna of U.S. Pat. No. 6,590,544, titled “Dielectric Lens Assembly for a Feed Antenna”. The lens antenna of U.S. Pat. No. 6,590,544 comprises a dielectric lens with a collimating part and an extension part, the collimating part and the extension part formed of a dielectric material, wherein the extension part comprises a substantially flat surface crossed by the axis of the collimating part, with at least one antenna element mounted on said surface, wherein the extension part of the lens consists of a plurality of dielectric substrates (see FIG. 2). The increase in directivity for a certain primary antenna element in such lens antenna is provided by selecting thicknesses and number of dielectric substrates, of which the extension part is comprised. The lens antenna of U.S. Pat. No. 6,590,544 is the closest prior art for the present invention.
However, the selection of lens extension length described in U.S. Pat. No. 6,590,544 is valid only for a specific primary antenna element. If the structure of the antenna element is changed, the selected thickness value will not be optimal. Therefore, the obtained optimal position of one antenna element is ineffective for another antenna element (having different radiation pattern properties in the lens body). In the invention of U.S. Pat. No. 6,590,544, antenna elements formed by two slots, spiral antennas, and an oscillating dipole with triangular arms are used. It is apparent that in order to maximize directivity of the lens antenna while using each of said antenna elements, the thickness and number of layers in the extension part of the lens may vary.
Furthermore, the lens antenna structure disclosed in U.S. Pat. No. 6,590,544 and other solutions described hereinabove, can be effectively used only in such millimeter wave communication systems where the required lens size is smaller than 10× wavelength in free space. For larger diameter lenses it can be shown that any modifications in the lens shape (with respect to the canonical hemielliptic with extension length equal to the lens focus) cause phase distortions in the field distribution on an equivalent circular aperture, leading to a change in signal phase in the peripheral areas of the aperture to the opposite value. This leads to a significant degradation of the lens antenna directivity. Therefore, in order to form lens antennas having a diameter of over 10×-20× wavelength in free space, lenses of standard hemielliptic shape with determined extension length (equal to the focal length of the lens) must be used. In this case, the use of antenna structure disclosed in U.S. Pat. No. 6,590,544 to maximize directivity becomes ineffectual.
Also an electronically steerable integrated lens antenna is disclosed in Alexey Artemenko et al., “Millimeter-Wave Electronically Steerable Integrated Lens Antennas for WLAN/WPAN Applications”, IEEE Transactions on Antennas and Propagation, vol. 61, no. 4, 1 Apr. 2013, pp. 1665-1671. The electronically steerable integrated lens antenna includes an extended hemispherical lens, four switched aperture coupled microstrip antenna elements, and a distribution circuit. There is also no possibility to increase lens antenna directivity since an array of standard microstrip patch antenna elements are used.
Further, US 2008/284655 A1 discloses a semiconductor antenna having antenna elements and a switching network formed in the same semiconductor die and configured to control activation of the antenna elements. Though the antenna elements are realized on a semiconductor die they have the same microstrip patch structure that cannot be configured to provide optimal lens illumination and, thus, maximum directivity and gain.
Furthermore, a dielectric lens antenna fed directly by the open end of a waveguide having a dielectric wedge is known form Fernandes C. A. et al., “Shaped Coverage of Elongated Cells at Millimetre Waves Using a Dielectric Lens Antennas”, Proceedings of the 25th. European Microwave Conference 1995. Bologna, Sep. 4-7, 1995, pp. 66-70. This document discloses the use of a hollow waveguide served at the same time as a feed waveguide. In this case the radiating opening of the waveguide is not capable to be optimized to have optimal illumination of the lens internal surface by incident electromagnetic waves that is caused by the fact that the feed waveguide cross-section size should be predetermined so to provide propagation of only one TE10 mode of the electromagnetic field. In that sense the feed waveguide is not effective and cannot be adapted to optimally illuminate lenses made of different dielectrics.
Therefore, it is an object of the present invention to increase directivity of a lens antenna when using lenses of any diameter, including large (>20× wavelength) diameters. It is another object of the present invention to provide high radiation efficiency and to improve impedance matching level in the lens antenna device. Achieving of said objects results in increasing the realized gain value of the lens antenna, and thus in increasing the effectiveness of millimeter wave communication systems.